Interference cancellation method, and receiver

ABSTRACT

An interference cancellation method and a receiver operating according to the method, in which multiple access interference cancellation of a received signal is performed and a confidence coefficient for estimates obtained from the received signal is calculated. In order to effect a reliable interference cancellation, the receiver utilizes the calculated confidence coefficients to control the multiple access interference cancellation.

This application is the national phase of international applicationPCT/F195/00553 filed Oct. 6, 1995 which designated the U.S.

BACKGROUND OF THE INVENTION

The present invention relates to an interference cancellation method ina data transmission system which utilizes a multiple access interferencecancellation method in which estimates of interfering signals aresubtracted from the desired signal, and in which method a confidencecoefficient is calculated for the estimates obtained from the receivedsignal.

In the design and implementation of data transmission methods, anessential problem is simultaneous transmission and reception of signalsof several simultaneous users, so that the signals cause as littleinterference to each other as possible. Owing to this fact and theavailable transmission capacity, several different transmissionprotocols and multiple access methods have been developed, the mostcommon of which in particular in mobile communication are the FDMA andthe TDMA methods, and lately also the CDMA method.

The CDMA (Code Division Multiple Access) system is a multiple accessmethod which is based on spread spectrum technology and whoseapplication in cellular communication systems has lately been initiatedalong with the earlier FDMA (Frequency Division Multiple Access) andTDMA (Time Division Multiple Access) technologies. The CDMA technologyhas several advantages over the earlier methods, such as simplicity offrequency planning and spectral efficiency.

In a CDMA method, the narrow-band data signal of the user is multipliedby a spreading code of much wider bandwidth to a relatively wide trafficchannel band. In the known experimental cellular network systems, thebandwidths used on traffic channels include, for example, 1,25 MHz, 10MHz and 25 MHz. In the multiplying process, the data signal spreads tothe whole band used. All users transmit simultaneously by using the samefrequency band. A separate spreading code is employed for eachconnection between a base station and a subscriber terminal equipment,and the signals from the users can be identified from each other in thereceivers on the basis of the spreading code of each connection. Anattempt is made for choosing the spreading codes so that they aremutually orthogonal, i.e. they do not correlate with each other.

Correlators or adapted filters in CDMA receivers implemented in aconventional way are synchronized with the desired signal, which isidentified on the basis of the spreading code. The data signal isreturned in the receiver onto the original band by multiplying it by thesame spreading code as in the transmission phase. The signals which havebeen multiplied by some other spreading code neither correlate norreturn to the narrow band in an ideal case. They thus appear as noisefrom the point of view of the desired signal. The aim is thus to detectthe signal of the desired user among several interfering signals. Inpractice, the spreading codes are not non-correlated, and the signals ofother users complicate the detection of the desired signal by distortingthe received signal. This interference caused by the users for eachother is termed as multiple access interference.

A data transmission method employing TDMA multiple access system hasseveral frequencies in use, each of which is divided into time slots inwhich the signals of the various users have been placed. Thus, each userhas a time slot of his own. As the frequency range reserved for thesystem is usually limited, the same frequencies must be used in cellslocated a certain distance away. If high frequency efficiency isdesired, this distance should be kept as small as possible. This resultsin different transmissions on the same frequencies interfering with eachother. Consequently, an interfering signal is heard in the receiver in acertain time slot in addition to the desired signal, which interferingsignal originates in some other connection using the same frequency.

The single user detection method described above in connection with CDMAis not optimum because, in connection with the detection, it disregardsinformation contained in the signals of other users. In addition, theconventional detection is unable to correct errors caused partly byunorthogonal spreading codes and signal distortion on the radio path. Anoptimum receiver takes into consideration the information contained inthe signals of all the users, and thus the signals can be detected in anoptimum manner by using, for example, a Viterbi algorithm. In the CDMAsystem, for example, an advantage of this detection method is that thesituation, as far as the receiver is concerned, resembles a single userCDMA system in which the multiple access problem does not exist. Forexample, the near-far problem, typical for CDMA systems, does not occur.The term near-far problem refers to a situation where a transmitterclose to the receiver covers the transmitters further away by itstransmission. The most serious weakness of the Viterbi algorithm is thatthe computational capacity it requires increases exponentially as thenumber of users increases. For example, a system of ten users in whichthe bit rate is 100 kbit/s by QPSK modulation would require 105 millionarithmetical operations per second for calculating the Viterbialgorithm. In practice, this constitutes a bar to the implementation ofan optimum receiver.

However, it is possible to approximate an optimum receiver by variousmethods. The prior art knows different kinds of methods for multiuserdetection (MUD). The best-known methods include a linear multiuserdetection, a decorrelating detector and a multistage detector. Thesemethods are examined in closer detail in the references Varanasi,Aazhang: Multistage detection for asynchronous code division multipleaccess communications, IEEE Transactions on communications, vol. 38, pp.509-519, April 1990, Lupas, Verdu: Linear multiuser detectors forsynchronous code-division multiple access channels, IEEE transactions onInformation Theory, vol 35, no. 1, pp 123-136, January 1989, and Lupas,Verdu: Near-far resistance of multiuser detectors in asynchronouschannels, IEEE Transactions on communications, vol 38, April 1990.However, these methods are also associated with operations, such asmatrix inversion operations, requiring a lot of computational capacity.

A second way for solving the problems caused by the multiple accessinterference is to use interference cancellation (IC) methods. In ICtype solutions, the purpose is to detect the users one by one, often inorder of magnitude, so that the influence of the signals of usersalready detected is eliminated from the received signal prior todetection of the subsequent user. As an example of such a solution,reference is made to the European patent publication 491668, applyingthe method described above in the CDMA cellular mobile communicationsystem. The interference cancellation methods are computationally moreefficient than the algorithms of the MUD type, but their performance isweaker particularly during poor reception conditions, such as a fadingmultipath channel, often having low signal levels.

Multiple access interference cancellation methods similar to the onesdescribed above can also be applied to TDMA systems. They do, however,have the deficiency of a deteriorating performance in case theinterfering signals have bad estimates. In the worst case, multipleaccess interference cancellation may even increase interference, if theinterfering signals are subtracted on the basis of wrong estimates.

BRIEF SUMMARY OF THE INVENTION

It is the purpose of the present invention to implement an interferencecancellation method by means of which the aforementioned problems can beavoided, and which does not make the receiver substantially any morecomplicated, and which can be applied to both the TDMA and CDMAmultiuser methods.

This object is achieved by a method set forth in the introduction, whichmethod is characterized in that confidence coefficients calculated forthe estimates of the interfering signals are utilized in the multipleaccess interference cancellation method.

In addition, the invention relates to a receiver of a data transmissionmethod, which receiver comprises means is for carrying out multipleaccess interference cancellation to the received signal, and means forcalculating a confidence coefficient for the estimates obtained from thereceived signal. The receiver of the invention is characterized in thatit comprises means for utilizing in the multiple access interferencecancellation the confidence coefficients calculated for the estimates ofthe interfering signals.

Thus, the basic idea of the invention is to take into account theconfidence coefficient of the estimate of the interfering signal priorto carrying out interference cancellation. Each estimated symbol can beweighted by a confidence coefficient which is calculated separately andwhich obtains values within the range 0 . . . 1. Unreliable interferenceestimates may be disregarded, because their subtraction from the signalmight even increase interference. Consequently, the interferencecancellation method achieves better results when those interferenceestimates that correspond well with the actual interference arecancelled. Furthermore, a signal having a low confidence coefficient canbe omitted from further processing, for example from the latter stagesof a multistage receiver, which results in a simpler structure and anincrease in computational efficiency.

Confidence coefficients have earlier been utilized in other parts of thereceiver, and in using a Viterbi decoder, for example, they arecalculated in association with detection, as well as, for example, insource decoding which has utilized coefficients calculated in channeldecoding. Of earlier targets of application for confidence coefficients,reference can be made to: U.S. Pat. No. 5,181,209, J. Hagenauer, P.Höher: A Viterbi algorithm with soft decision outputs, Proc. of IEEEGlobecom 1989, Dallas, Tex., and P. Höher: TCM on frequency selectivefading channels: comparison of soft output probabilistic equalizers,Proc. of IEEE Globecom 1990. The term soft output is in somepublications used in a different sense than in this application. Thisapplication uses said term to refer to a confidence coefficient ofgrouping while, for example, the publication D. Brady, J. Catipovic: Anadaptive soft-decision multiuser receiver for underwater acousticalchannels, 26th Asilomar Conference on Signals, Systems & Computers, 1992uses it in reference to heuristic scaling.

The method according to the invention enables, due to the more efficientinterference cancellation, a narrower frequency re-use spacing in TDMAsystems, for example, which in turn increases system capacity.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

In the following, the invention will be described in closer detail withreference to the examples of the accompanying drawings, in which

FIG. 1 is a block diagram illustration of the structure of a receiveraccording to the invention,

FIG. 2 illustrates in closer detail, by means of a block diagram, theimplementation of interference cancellation method according to theinvention,

FIG. 3 shows, by means of a block diagram, an alternative structure of areceiver according to the invention, and

FIG. 4 illustrates the structure of a multistage receiver.

DETAILED DESCRIPTION OF THE INVENTION

The confidence coefficient in this invention is associated with detectedsymbols. It depends on the distribution of the received signal, whichdistribution in turn depends on, among other things, the symbolalphabet, transmission path and signal processing within the receiver.The distribution of the received signal particularly depends on thestrength of multipath propagated signals, channel status, andcorrelations between the received symbols. In CDMA systems, interferencecancellation methods are above all based on optimum or sub-optimumdecision rules which take into account the correlations between thesymbols of the various users.

The confidence coefficient can be generated by means of, for example,maximum a posterior (MAP) or MUSE criteria. Typically, the detectionconfidence is judged by estimating the probability of an erroneousdecision:P _(ER) =Prob(Ŝ≠S _(k) |r),in which S_(K) denotes the symbol transmitted, Ŝ is the estimated symboland r the signal supplied to the detector. The symbols are naturallydefined by a way appropriate for the application. It is, however,included in the symbol alphabet generally used in the transmitter. Itmay also describe a multidimensional symbol vector whose kth coordinateis included in the symbol alphabet of the kth user. Generally speaking,then, the confidence coefficient is a function of the distribution ofdetected symbols or parameters of the distributions.

The received low-pass filtered signal can generally be represented bythe equation${{r_{m,n}\quad(t)} = {{S_{n}\quad{\sum\limits_{k = {- \infty}}^{\infty}\quad{x_{n,k}\quad q_{m,n}\quad\left( {t - {kT}} \right)}}} + {n_{m,n}\quad(t)}}},$in which T is the duration of the signal, X_(n,k) is the source symbolof the nth user at the moment of time k, andq _(m,n)(t)=c _(T)(t−t _(n)){circle around (×)}h _(m,n)(t){circle around(×)}c _(R)(t)is the convolution of the impulse responses or the transmitter filter,multipath channel and receive filter, respectively, on the mth diversitybranch from the nth source. The total signal received by the mthdiversity branch is then${{r_{m}\quad(t)} = {{\sum\limits_{n = 0}^{N}\quad{r_{m,n}\quad(t)}} + {n_{m}(t)}}},$in which n_(m)(t) denotes noise. In the simplest case m=1.

It is assumed without effecting generality that the index of the desireduser is zero, n=0. The method can correspondingly be applied to theother users. The interfering signal components are divided into twoparts, the dominant components Nd and noise components Nn, and N Nd+Nn.In the receiver, attempt is made for removing the influence of thedominant noise components. The noise components comprise thoseinterfering signal components that are weak as to their strength, andwhose effect in the interference cancellation may be ignored byconsidering them as noise-like interference. The total signalillustrated in the formula above can be shown in the form$\begin{matrix}{{r_{m}\quad(t)} = {{r_{m,o}\quad(t)} + {\sum\limits_{n = 1}^{Nd}\quad{r_{m,n}\quad(t)}} + {\sum\limits_{n = {{Nd} + 1}}^{N}\quad{r_{m,n}\quad(t)}} + {n_{m}\quad(t)}}} \\{{= {{r_{m,o}\quad(t)} + I_{Nd} + I_{Nn} + {n_{m}\quad(t)}}},}\end{matrix}$in which I_(Nd) comprises the sum signal of the dominant interferingsignal components and I_(Nn), the sum of interference componentsclassified as noise components. In the receiver, detection is desired ofthe transmitted symbols x_(n,k) from the received signal, which isillustrated above. If the interference components I_(Nd) and I_(Nn) aresmall, the reception can be carried out by conventional single userreception methods, as is done in the GSM system, for example. As theco-channel interference increases as a result of reducing frequencyre-use distances, more efficient methods will be required.

In the preferred embodiment of the method according to the invention,interference cancellation is carried out by subtracting from the desiredsignal the estimates of the dominant interfering signal, the estimatesbeing weighted by the confidence coefficient of the estimates. Thecalculation for the estimates of the dominant interfering signal can berepresented byÎ _(Nd) =Ŝ _(n) Σ{circumflex over (x)} _(n,k) {circumflex over (q)}_(m,n)(t−kT)g _(n,k)where g_(n,k) is the confidence coefficient of the estimate of theinterfering signal, and{circumflex over (q)} _(m,n) =c _(t)(t−{circumflex over (t)}_(m)){circle around (×)}ĥ_(m,n)(t){circle around (×)}c _(t)(t)is the convolution described earlier, where ĥ_(m,n) and {circumflex over(t)}_(m) are channel estimates.

The method can in a simple way be applied by, for example, detecting thesymbols of all the dominant interfering signals and calculating aconfidence coefficient for the estimates according to how accurate theobtained estimate is considered to be. Following this, the interferingsignal is regenerated, multiplied by the confidence coefficient andsubtracted from the received transmission. The confidence coefficientmay obtain values between the range [0,1] so that if the estimate isconsidered as unreliable the coefficient value is close to zero, and ifthe estimate is considered as reliable the coefficient value is close toone. If all the interference estimates have a high confidencecoefficient, it is possible to cancel the interference almost entirely,but if they all have a low confidence coefficient there is practicallyno interference cancellation at all. This way it is possible to avoid asituation where an interference calculated on the basis of a wrongestimate is subtracted from the signal, leading in fact to an increasein interference. In the solution according to the invention, theoperation of the receiver automatically alternates between that of asingle user receiver and an interference cancellation receiver dependingon confidence coefficients obtained at any given moment of time.

The confidence coefficient may be calculated, for example, inassociation with a channel decoder. If a Viterbi decoder is employed bythe receiver, the confidence coefficient is obtained without anyadditional procedures, because it is calculated anyway and utilized inenhancing the operation of the source decoder.

The method according to the invention can also be applied to amultistage receiver structure in which interference cancellation methodutilizing confidence coefficients according to the invention is appliedto the first or latter stages. On every cycle of multistage detection, anumber of interference terms are separately estimated, whichinterference terms are subtracted from the received signal according to,for example, the following formulas, again assuming that n=O:Î _(Nd) ^((i)) =Ŝ _(n) ^((i)) Σ{circumflex over (x)} _(n,k)^((x))(t−kT)g _(n,k) ^((i))r _(m)(t)^((i)) =r _(m,o)(t)+I _(Nd) +Î _(Nd) ^((i)) +n _(m)(t)in which symbols {circumflex over (x)}_(n,k) ^((i)), {circumflex over(q)}_(m,n) ^((i)) defining channel parameters and the confidencecoefficients ĝ_(n,k) ^((i)) have been estimated by the desired methodfrom the signal Î^((i−1)). Typically, initial values are at firstestimated for the channel parameters {circumflex over (q)}_(m,n) ⁽⁰⁾,which are used in detection of symbols {circumflex over (x)}_(n,k) ⁽⁰⁾.At the initial stage these variables are estimated from the signalsubject to interference, which results in their reliability beingtypically bad.

In the estimation, a training sequence can be utilized, i.e. a knownsymbol sequence in the received signal, improving the performance ofchannel estimation. Thereby, as the symbols are known, it is possible toa priori set the confidence coefficient value to 1.

As soon as {circumflex over (q)}_(m,n) ^((i)) has been calculated, thesymbols {circumflex over (x)}_(n,k) ^((i)) can be detected by using theselected decision rule for the selected symbol alphabet. The symbolalphabet may, for example, be BPSK, QPSK or some other multidimensionalsequential symbol. In the CDMA system, {circumflex over (x)}_(n,k)^((i)) may, for example, be a modulated spreading sequence.

FIG. 1 is a block diagram illustration of the structure of a receiveraccording to the invention. The receiver of the invention illustrated inthe figure comprises an antenna 10 by means of which the received signalis fed to the radio frequency parts 11 in which the radio frequencysignal is transferred onto an intermediate frequency. From the radiofrequency parts, the signal is fed to an analog/digital converter 12 inwhich the received analog signal is converted to digital form. Theconverted signal is further fed to a detection means 13 in whichestimates of the received signal are detected for the received symbols.In the receiver according to the invention, the confidence coefficientfor the detected symbols is also calculated in the means 13. Theestimates and the calculated confidence coefficients are further fed toan interference cancellation means 14 in which, according to thepreferred embodiment of the invention, the interference estimates aremultiplied by the confidence coefficient and subtracted from the desiredsignal. The purged signal is further fed to the other parts of thereceiver. The receiver of the invention can be used in both the basestation and the subscriber end terminal in a cellular mobilecommunication system. Naturally, the receiver according to the inventionalso comprises other components than those described above, such asfilters and amplifiers.

The interference cancellation according to the invention canadvantageously be carried out in a detection-interference cancellationblock of FIG. 2 stage by stage. The received signal 28, which has beenconverted into digital form, is fed to the detection means 20 in whichinitial estimates are calculated for the received signals. In means 21,a confidence coefficient is calculated for the estimates, the value ofwhich confidence coefficient describes how well the estimate is assumedto represent the transmitted symbol. The detected signals areregenerated again in means 22, after which the regenerated signals aremultiplied by corresponding confidence coefficients in a multiplyingmeans 23. Following this, the interfering signals that have beenregenerated and weighted by coefficients are fed as a negative input toa summing means 24 whose second, positive, input is the originalreceived signal delayed by delay means 25, and in which summing means 24the weighted interfering signals are subtracted from the originaltransmission. Thus, the purged signal 26 is further fed to the seconddetection means 27 in which the desired signal is estimated again.

FIG. 3 illustrates a second possible way to implement a receiveraccording to the invention. The receiver of the figure comprises means30 by which the received signals are divided in several groups eachcomprising at least two signals. The receiver further comprisesdetection means 31, 35, 39 for carrying out in each group a simultaneousdetection for the signals from the received transmission by any knownmultiuser detection method. In detection means 31, 35, 39 a confidencecoefficient is calculated for the detected signals. Said calculatingmeans can be comprised in connection with said detection means orseparate from them. The receiver also comprises means 32, 36, each forregenerating the detected signals again and multiplying the signals bythe corresponding confidence coefficients. Each of means 32, 36 isconstituted by an assembly identical to means 21, 22 and 23 of FIG. 2.The group detection means 31, 35, 39 have been grouped so that eachgroup is consecutively detected so that in the summing means 34, 38 ofthe receiver it is possible to subtract the signals from means 32 and36, respectively, weighted by the confidence coefficients of the groupsalready detected from the received transmission delayed by delay means33 and 37 respectively prior to detecting the subsequent group. Areceiver structure utilizing the group detection illustrated above is incloser detail described in the Finnish patent application 943196 whichis herein incorporated as reference.

FIG. 4 illustrates the structure of a multistage receiver by means of ablock diagram. The multistage receiver comprises several detectionstages 40, 41, 42 commonly connected in series. In such a case, thelatter detection stages utilize the detection carried out already at theearlier stages, especially as interference cancellation is being carriedout. In the multistage receiver of the invention, one of the earlierdetection stages 40, 41, 42, at least, comprises the means 14, describedabove, to utilize in the multiple access interference cancellationconfidence coefficients calculated for the estimates of the interferingsignals.

Although the invention is above described with reference to the examplesof the attached drawings, it is obvious that the invention is notrestricted to them, but it may be varied within the inventive idea ofthe attached claims.

For example, the signal regenerated in the CDMA systems is narrow-bandif it is subtracted from a narrow-band signal. Generally speaking,regeneration and subtraction should be matched to the method used forprocessing the received signal in the receiver prior to the interferencecancellation utilizing confidence coefficients according to theinvention.

1. A method for canceling signal interference in a TDMA datatransmission system, comprising: calculating estimates of interferingsignals in the TDMA data transmission system; subtracting the estimatesof the interfering signals from a desired signal; calculating aconfidence coefficient in a soft-output channel decoder in the TDMA datatransmission system, the confidence coefficient having a value in arange greater than 0 and less than 1, for the estimates of theinterfering signals based on a received signal; forming an estimate ofeach interference signal as a product of a channel estimate and outputof the soft-output channel decoder; subtracting the formed estimatesfrom the received signal; dividing received signals into several groupsin a receiver, each group comprising at least two signals; and carryingout in each group simultaneous detection of the signals from a receivedtransmission; wherein each group is detected consecutively; wherein thesignals of groups already detected are regenerated and subtracted fromthe received transmission prior to detecting a subsequent group; whereinthe signals detected prior to the subtraction from the receivedtransmission are weighed by a confidence coefficient calculated for eachreceived signal.
 2. A receiver of a TDMA data transmission system,comprising: means for carrying out multiple access interferencecancellation for a received signal in the TDMA data transmission system;means for calculating a confidence coefficient in a soft-output channeldecoder in the TDMA data transmission system, the confidence coefficienthaving a value in a range greater than 0 and less than 1, for estimatesof the received signal; means for calculating confidence coefficientsfor estimates of interfering signals; means for forming an estimate ofeach interference signal as a product of a channel estimate and outputof the soft-output channel decoder; means for dividing received signalsinto several groups, each comprising at least two signals; means forcarrying out, in each group, a simultaneous estimation and detection forthe signals from a received transmission and for consecutive detectionof each group; means for regenerating the signals of groups alreadydetected and for weighing the signals already detected by a confidencecoefficient calculated for each of the received signals; means forsubtracting the weighted estimates from the received signal; and meansfor subtracting the signals of groups already detected from the receivedtransmission prior to detection of a subsequent group.
 3. A receiver ofa TDMA data transmission system, comprising: a multiple accessinterference cancellation mechanism to carry out multiple accessinterference cancellation for a received signal in the TDMA datatransmission system; a confidence coefficient calculating mechanism tocalculate a confidence coefficient in a soft-output channel decoder inthe TDMA data transmission system, the confidence coefficient having avalue in a range greater than 0 and less than 1, for estimates of thereceived signal; a utilizing mechanism to utilize the confidencecoefficient calculating mechanism to calculate confidence coefficientsfor estimates of interfering signals; a forming mechanism to form anestimate of each interference signal as a product of a channel estimateand output of the soft-output channel decoder; a signal dividingmechanism to divide received signals into several groups each of whichcomprises at least two signals; a signal estimating and detectingmechanism to carry out in each group a simultaneous estimation anddetection for the signals from a received transmission and toconsecutively detect each group; a signal regenerating and weighingmechanism to regenerate the signals of groups already detected and toweigh the signals already detected by a confidence coefficientcalculated for each of the received signals; a signal subtractingmechanism to subtract the signals of groups already detected from thereceived transmission prior to detection of a subsequent group; and asubtracting mechanism to subtract the weighted estimates from thereceived signal.